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Coin Cell Power for MSP430

Charles Gervasi
Expert 2270 points
Other Parts Discussed in Thread: TPS61097-33, TPS61220, TPS61221

I have a board with an MSP430 and some LEDs.  The voltage is 2.5V-3.0V.  Peak current will be a max of  50mA for 50ms supplied by a supper cap.  Ave current will be less b/c everything will sleep most of the time.  I'm running the board with a Li coin cell.  I'm trying to decide whether to use a boost converter or just a a simple circuit connecting the battery to the super cap.  That way I'll be able to suck a little more power out of the coin cell when its voltage falls below 2.5V, but it adds complexity and inefficiency.  There is an app note for doing this with a buck converter that spends most of its time in shut down, but if I do a switcher it will be a boost.

This is an engineering problem I'm working out.  My question for TI and the E2E community is has anyone had experience powering a device with a high peak-to-ave current ratio on a Li coin cell.  Is the ability to pull power from a Li coin cell at 2V worth the inefficiency of the switcher? 

  • 0
    TI__Guru**** 150930 points

    I believe the TPS61097-33 is perfect for this application.  There is a 3V version in work.

    Another option is the TPS61220.

    Either solution allows you to increase the battery life of your system by drawing all the power out of your battery, even at its lower voltages.  With a fresh battery, the switcher remains off and passes through the battery voltage without decreasing battery life.

  • 0 in reply to Chris Glaser
    Prodigy 160 points

    There is no way a lithium coin cell battery can provide the input current that the TPS61097-33 needs. Even in discontinuous mode, the peak inductor current wants to get to 200mA (see figure 17 in the datasheet). A lithium coincell is incapable of providing 200mA without the terminal voltage dropping to 1V or less. I think this would be very bad for the coin cell indeed.

     If I'm wrong, please let me know!  Thank you.

  • 0 in reply to John Araujo
    TI__Guru**** 150930 points

    Yes, the TPS61097 wants to get up to 200 mA in a switching cycle.  But it switches very fast.  So, that current comes from the input capacitor.  The input capacitor delivers the higher peak currents, while the coin cell delivers the lower average input current.

  • 0 in reply to Chris Glaser
    Prodigy 160 points

    Thank you for pointing that out. If the TPS61097 has an eval board, I might want to pick one up and give it a try.

  • 0 in reply to Chris Glaser
    Prodigy 160 points

    Thank you. I'll give it a try!   Any idea when the 3.0V version will be available?

  • 0 in reply to John Araujo
    TI__Genius 14890 points

    John,

    The 3.0V version is still in development.  We don't have a firm plan on release yet.  What are your timing requirements?

    Thanks,

    Dick

  • 0 in reply to Dick Stacey
    Prodigy 160 points

    I'm starting to develop a new product now. The 3.3V part may be fine. I'm looking at a product that we'd like to operate from a 3V lithium coin cell. However, there may be an ADC in the design that is only specified to 2.7V. 2.7V is getting dead  for a 3V coin cell, but they are specified down to 2.0V. I hate to throw that all away, but I'm also not sure adding a step-up is the best idea either. I'd rather see if I can run this product from a 3.6V lithium battery, but that has case implications. In any event, I do want to see how the TPS61097 operates with a coin cell input. It might have future applications beyond this current product. Thanks!

  • 0 in reply to Chris Glaser
    Prodigy 10 points

    I've been facing an issue with boosting a Li coin battery to 3.3V with TPS61221.  I have a custom PCB which I followed the PCB layout on the TPS61220 datasheet.  With a power supplier there is no problem, but when I implemented a coin battery (Panasonic CR1025) the output turns out to be ~0.8V.  Do you think the current limitation of the coin battery is causing this problem?  If there is any other cause, please let me know.  Thank you!

  • 0 in reply to Daiki Itoh
    TI__Guru**** 150930 points

    Yes, since the only item that you changed was your input source, this must be the source of your issues.

    Coin cells have a rather high source impedance.  This means that their voltage drops as you draw current from them.  To overcome this, more input capacitance is required.  I would try adding a 100uF ceramic Cin and see if your problem is solved.

  • 0 in reply to Daiki Itoh
    Expert 2270 points

    Daiki Itoh said:

    I've been facing an issue with boosting a Li coin battery to 3.3V with TPS61221.  I have a custom PCB which I followed the PCB layout on the TPS61220 datasheet.  With a power supplier there is no problem, but when I implemented a coin battery (Panasonic CR1025) the output turns out to be ~0.8V.  Do you think the current limitation of the coin battery is causing this problem?  If there is any other cause, please let me know.  Thank you!

    I found Li coin cells had a good amount (can't remember the figures) of internal resistance, but that that resistance increased about 50 times when the cell was about half depleted.  I abandoned using Li coin cells on that project b/c while it would have worked I could only extract a third of the power from the battery.  People would be throwing away batteries with over half their life left. 

    Maybe you could put a current sense resistor on the output.  Maybe the inrush current to charge the caps and whatever is high enough to pull the li ion cell voltage below the switcher's under-voltage lockout. 

    Also, see if I/O of some processor is connected to the battery and current flows through the protection diodes, supplying current to the I/O rail, before the switcher has a chance to start.  If you have transistors that connect the rail to multiple voltage sources, this can cause a problem like this. 

    If inrush current is the problem, maybe you could add some bulk capacitance in parallel with the battery.